This invention relates to the stabilization of polyacrylonitrile compositions. More particularly, the invention relates to the stabilization of polyacrylonitrile compositions containing a low-boiling solvent, which compositions are particularly useful for the formation of shaped acticles such as fibers and films and which have a tendency to develop color either upon standing or upon the application of heat.
The terms "polyacrylonitrile" and "acrylonitrile polymers" as used herein refer to homopolymers as well as copolymers of acrylonitrile containing at least about 85 percent by weight acrylonitrile and up to about 15 percent by weight of at least one other ethylenically unsaturated compound copolymerizable with acrylonitrile.
The utility of acrylonitrile polymers is wellestablished, particularly for the preparation of such shaped articles as fibers and films. However, preparative techniques generally are restricted to dry spinning and wet spinning from solutions having a polymer concentration up to about 25 to 40 percent by weight. Conventional spinning technology, dry spinning in particular, has utilized relatively high boiling solvents, e.g., boiling above about 100.degree. centigrade, for forming spinnable dopes of acrylonitrile polymers. The formation of shaped articles by a melt extrusion process, that is, a process in which the polymer is melted and the molten polymer extruded through a die or spinnerette into an inert medium in contradistinction to dry and wet spinning methods in which a solution of the polymer is extruded through a die or spinnerette, is not practical because of the relatively high melting temperatures of acrylonitrile polymers containing at least 85 percent by weight acrylonitrile and because of the tendency of such polymers to decompose before or during melting.
Acrylonitrile polymers having an acrylonitrile content of at least 85 percent generally are insoluble in the more common solvents. Whenever suitable solvents have been found, however, the application of heat usually is necessary in order to effect solution. The application of heat to effect solution usually results in the development of a pale yellow color in the resultant solution, which color generally darkens and becomes brown with time. Even without the application of heat or upon removal of heat after a solution has been obtained, color develops in solutions upon standing for prolonged periods of time. Color development is more pronounced at elevated temperatures and in the presence of bases or other strong nucleophiles. Color development is most severe in N,N-dimethylformamide solutions such as those employed in dry spinning. Obviously, any color developed in polymer solutions or spinning dopes will be carried over into the products formed therefrom.
The mechanism responsible for color formation and the nature of the chromophores involved still are uncertain. The color may be caused by the presence of metal ions such as iron, copper, and manganese in the polymer solutions. Impurities other than metal ions present in the solvents also have been cited as a cause. The employment of amides, such as N,N-dimethylformamide, as solvents may result in the thermal decomposition of the solvent to give amines which in turn may cause color formation. Or, color formation may be caused by conjugation of carbon -- nitrogen double bonds derived from the cyanide groups in the polyacrylonitrile. Such conjugated double bonds conceivably may result from a cyclization reaction initiated perhaps by oxidation products, such as hydroperoxides, in the polymer. The resultant conjugated structures would be colored because of the low energy electronic transitions possible in the resonating double bond system. Regardless of the mechanism which gives rise to color in solutions of acrylonitrile polymers, e.g., spinning dopes, the presence of color in polymer solutions results in shaped articles which are colored. Such coloration is undesirable for aesthetic reasons and contributes to product non-uniformity.